The present invention concerns a thermal printing device comprising a frame, a thermal printhead flexibly mounted on the frame, a driving roller carried by a cover and integral with a gear assembly, the printhead resting on the roller when the device is in a printing position, and two orifices arranged in each lateral side of the frame, in which ends of a rotation axis of the roller can be inserted, in which the loading and unloading of the paper, and in particular the closure Of its frame, are facilitated.
Printing mechanisms generally consist of a printhead affixed on a frame. The thermal printhead includes a ceramic support bearing the line of heating dots and the chips controlling their power supply. The printhead can pivot relative to the frame, most commonly along an axis parallel to one longitudinal side of the frame. A roller is integral with the frame such that its longitudinal axis is also parallel to one longitudinal side of the frame. The printhead is held in contact with the roller by a spring. The position of the roller must be controlled perfectly in order to obtain good alignment with the thermal head; the printing support usually consists of a roll of paper, one side of which is heat sensitive. This printing roller is driven in rotation by means of a driving roller which is, for its part, actuated by a system of gears and by a small electric motor.
One of the primary difficulties in the use of these mechanisms is performing the introduction of the end of the roll of thermal paper between the roller and the thermal printhead. This loading operation is currently accomplished in several ways.
One simplified system uses the printhead as a cam to lock the head in position.
Another system consists in moving the head away from the roller by means of a lever. Such a system is described, for example, in the document FR-A-2 737 152 and in the document FR-A-2 737 153. The thermal printing mechanism described in these documents includes a frame, a roller, a printhead, means forming a spring to tension the printhead against the roller, and a cam mounted pivotably around one axis of the frame to move the printhead away from the roller. The major disadvantages of these devices are the complexity of the lifting mechanism (cam and lever), the necessity of an additional sensor, usually a switch, to detect the lifting, and the management of the electronics involved.
There are also devices in which the printhead is separated using an element of the frame or a pivoting cover whose movements are relatively complex, as it is first necessary to remove the roller integral with this pivoting element from the printhead before being able to pivot the assembly when the roller is no longer in contact with the head, thus freeing the space to load the paper.
Other devices include means to lock the printhead on the roller, which, when deactivated, for example, by pressing on a mechanical part, release the printhead from the lock and enable opening the mechanism.
A thermal printing device for paper having a thermal printhead working by contact with a roller for driving the paper has been described, wherein the printhead is flexibly supported by a fixed part of the frame whereas the driving roller is carried by a movable frame element articulated on the fixed frame and forming a cover for access to the compartment of the device designed for the paper roll, with the printing device being such that the contact zone of the line of heating dots of the printhead on the paper driving roller is located behind the diameter of the roller passing through the articulation of the movable frame on the fixed frame when the cover is closed in the direction of closing the cover.
This device presents the major disadvantage of kg the closing of the mechanism with the position of the axis of rotation of the cover relative to the frame.
This device also presents the disadvantage of not being able to permit use of the mechanism in reverse operation and, if the user pulls on the end of the roll of paper or attempts to cut it using a manual paper cutter, this causes a loss of engagement of the gears, which translates into unpleasant noise and the opening of the cover.
Also known by EP-0 765 761 is a thermal printing mechanism in which lateral orifices of the frame receive the extremities of the axis of the roller. Claws enable closing the cover, in order to avoid that the head become displaced during closing of the cover and move back to its point of minimal pressure, which is never a printing position.
Further, Japan Patent Abstracts Vol. 098 No. 10 of Aug. 31, 1998 discloses a thermal printing mechanism including lateral orifices; the arrangement is such that it is necessary to close the cover to enable the printer to operate by preventing the bearings which carry the roller to rotate around their own axes relative to the frame.
The known devices of the prior art, with the exception of the first mentioned above, are relatively complex in terms of their implementation, a situation which poses problems of mechanical tolerances, particularly when one wishes to use compact devices to reduce their bulk.
Consequently, an objective of the invention is to remedy the disadvantages mentioned above.
One object of the invention is to enable locking and unlocking the roller against the printhead so as to enable easy introduction of the end of the roll of thermal paper between the roller and the printhead.
Another object of the invention is to enable locking and unlocking the roller against the printhead without complicating the device with cam or lever means.
To that end, the invention concerns a thermal printing device wherein each orifice has a profile comprising a shoulder forming a position of unstable equilibrium of the axis of the roller during movement of said axis in said orifices, and said shoulder of unstable equilibrium is arranged between two positions of stable equilibrium of the axis and the roller, at an entry of a top part of the orifices and at a bottom of the orifices in a closed, locked position of the roller, respectively.
The cover, consisting of one or a plurality of articulated elements, supports the roller and enables bringing it to the opening of the top part of the orifices and permits its free movement inside said orifices. The parts of the orifices opposite the printhead are homothetic to each other relative to the axis of rotation of the roller when it is engaged in the base of the orifices.
A first advantage of the invention is that the mechanism can be extremely narrow with a simple clipping system.
The device is such that the parts of the orifices opposite the printhead have a shape which guides the roller from the position of entry on the frame to the printing position such that at the time of movement of the roller, it passes through a position of unstable equilibrium caused by a shoulder generating a friction point and instability point of the roller during movement of its axis in said orifices when it is subject to the pressure of the printhead.
The device is such that one orifice is wider than the other.
The device is such that the shape of the bottom section of the orifice follows a radius of the gear assembly driving the gear assembly integral with the roller.
The device further has a part which moves into place over the gears to protect them and serves to align the roller relative to the gears.